Method of removing mercaptans from a liquid mixture of hydrocarbons containing low-boiling and high-boiling mercaptans



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METHOD OF REMOVING MERCAPTANS FROM A LIQUID MIXTURE OF HYDROCARBONS CONTAINING LOW BOILING AND HIGH BOILING MERCAPTANS 2 Sheets-Sheet 1 Filed Jan. 26, 1945 5 M n a m E M @w 5 w T M w m C M W W C|N nu 3 2 I 0 0 O 0 KERNWEE \fi Gm Ex kokwkwzmwmm R muss wibmmw 1948- s. P. CAULEY METHOD OF REMOVING MERCAPTANS FROM A LIQUID MIXTURE OF HYDROCARBONS CONTAINING LOW BOILING AND HIGH BOILING MERCAPTANS 2 Sheets-Sheet 2 Filed Jan. 25, 1945 INVENTOR QNkoDvMRRb MWRR Qkbwhw MORE. kwk

captans from hydrocarbon mixtures.

Marnop or REMOVING maaoar'mns mom a romo mx'rcne or no CARBONS CONT D GH-BOILING Stephen P. Cauley, Garden City, N. Y., poor to Socony-Vacuum Oil Company, Incorporated, a corporation of New York Application January 26, 1945, Serial No. 574,759-

3 Claims. (Cl. 196-30) The present invention relates to the removal of mercaptans from hydrocarbon mixtures containing acidic sulfur compounds and, more par ticularly, to the removal of mercaptans from petroleum oil fractions.

It has been the practice in the petroleum oil industry for many years to remove undesirable acidic sulfur compounds, such as mercaptans, from petroleum oil fractions by contacting the contaminated oil with aqueous caustic solutions.

- This practice lead to the discovery that the solubility of mercaptans in aqueous caustic solutions is increased when the caustic solution contains a substance such as isobutyric acid, cresols, alkanolamines and others well known to those skilled in the art as soiutizers. More recently it was discovered that prior art regeneration of the caustic solution by steaming or air blowing could be improved by incorporating in the aqueous caustic solution a soluble organic oxidation promoter with or without solutlzers whereby the aqueous caustic solution can be regenerated by oxidation of the mercaptan-sulfur to disulflde sulfur and the mercaptan-smfur separated from the regenerated caustic solution in the form of disulfidesulfur. A further development of the latter development was the discovery that the soluble organic oxidation promoter is oxidatively inactivoted or destroyed in the absence of a minimum amount of mercaptan-suliur or other protector for the soluble organic oxidation promoter. It has now been discovered that the protective emciency of mercaptan-suliur for the soluble organic oxidation promoters is dependent upon the molecular weight of the mercaptans involved and that the oxidation rate of mercaptan-sulfur is dependent upon the presence or absence of solutizers. Consequently, the improved results obtained by regenerating fat aqueous caustic solutions by oxidation of mercaptan-sulfur to disulfide-sulfur in the presence of soluble organic oxiof the present invention to provide a process for the removal of mercaptans from hydrocarbon mixtures in which the greater protective value of higher molecular weight mercaptans for soluble organic oxidation promoters is utilized. This invention also has as an object the provision of a method for removing mercaptans from hydrocarbon mixtures employing in one stage an aqueous caustic solution of high absorptive power to extract the less reactive mercaptans and in another stage an aqueous caustic solution of less absorptive power to extract the more reactive mercaptans. Other objects and advantages will become apparent from the following description taken in conjunction with the drawings in which Figure 1 is a graph illustrative of the relative regeneration rates of individual mercaptans, and

Figure 2 is a flow-sheet illustrating in a more or less diagrammatic manner one means for accomplishing the objects of this invention.

One of the most economical means for removing mercaptans from petroleum distillates involves extracting the mercaptans by contacting the oil with aqueous alkali metal hydroxide solution containing soluble organic oxidation promoter. The aqueous solution may or may not contain mercaptan solubility promoters. The aqueous solution is separated and thereafter the solu tion is regenerated by blowing an oxidizing gas. particularly an oxygen-containing oxidizing gas, through the aqueous solution to convert the mercaptans to disulfldes. The disulfides are insoluble in the aqueous solution and are readily separated by a settling operation.

The successful operation of the typ of operation outlined immediately herelnbefore is limited to an appreciable extent by several factors. When the aqueous solution containing organic oxidation promoter and mercaptan is regenerated by conversion of mercaptans to dlsulfides by blowing oxidizing gas through the solution the prodation promoters can be improved even more by applying the most recently discovered facts con-- cerning the mechanism of the oxidation regeneration method to problem of removing mercaptans from petroleum oil fractions.

It is an object of the present invention to provide an improved method for removing mer- It is another object of this invention to provide an improved method ior removing mercaptans from hydrocarbon mixtures in which advantage is taken of the fact that the rate of oxidation of mercaptan to disulfide is dependent at least in part upon the presence of solutizers. It is a further object moter is inactivated or destroyed when the mercaptan concentration is reduced to below a minimum value which is normally about 0.1 per cent to about 0.3 per cent by weight. However, the protection afforded to the oxidation promoter by the residual mercaptan content of the aqueous solution is dependent upon the molecular weight of the residual mercaptans dissolved in th aqueous solution. For example, to protect the organic oxidation promoter, such as tannin, present in an amount of about 1 per cent by weight in an alkaline solution containing only methyl merper cent by weight to prevent excessive depreciation of the organic oxidation promoter. On the other hand. the organic oxidation promoter in similar alkaline solutions containing only butyl mercaptide and hexyl mercaptide can be adequately protected by a concentration of 0.2 per cent and 0.1 per cent by weight respectively. The foregoing makes it manifest that adequate protection of an organic oxidation promoter in such alkaline solutions would involve determining the minimum protective mercaptide concentration for each new mixture of mercaptans which was encountered in actual operation. I The problem of determining the minimum protective mercaptide concentrations for various mercaptan mixtures would not be an insurmountable task if all mercaptans were converted to disulfldes with equal facility. However, the fact is that the rate of conversion of mercaptans to disulfldes varies for different mercaptans.

The characteristics of oxidative regeneration of the type discussed hereinbefore are such that certain mercaptans (mercaptides) are regenerated more rapidly than others. The graph Figure 1 depicts the relationship of rate of oxidation to molecular weight of the mercaptide in a particularly clear manner. This graph clearly shows that as the molecular weight of the mercaptan increases the rate of oxidation or conversion to t as rapidly as C1 mercaptans. Consequently, even if the minimum protective value of a mixture of C1 to Ca mercaptans were determined the value would not be valid after a period during which the ratio of heavy mercaptans (C4 to Co and heavier) to light mercaptans (C1 to Ca) had changed due to the greater speed at which the heavier mercaptans are converted to disulfldes and removed from the aqueous alkaline solution. .A third factor also enters into the problem. Alkaline solutions containing mercaptan solubility promoters and alkaline solutions containing no added mercaptan solubility promoters are used to remove mercaptans from petroleum distillates such as gasoline. The rate at which various mercaptans (alkali metal salts thereof, 1. e., mercaptides) are converted to disulfldes by oxidizing gas in the presence of organic oindation promoter varies considerably not only with the molecular weight of the mercaptide as pointed out hereinbefore but also with the concentration of mercaptan solubility promoters present and the particular solubility promoter. This is clearly established by the following tabulation in which the rates of conversion of mercaptan-sulfur to disulfide-sulfur for various mercaptans in a solution devoid of solubility promoter and in solutions containing difierent solubility promoters are compared. 7

1 Bate=per cent 8(RSH) converted to B-S-S per minute. Tannln=i per cent Tannin as Chestnut Tannin extract.

The data presented in the table clearly show that all mercaptans are converted to disulfldes in the presence of organic oxidation promoters in simple aqueous alkali metal hydroxide solutions more rapidly than in similar solutions conessentially free of hydrogen sulfide containing mercaptans is extracted with an aqueous alkaline solution of relatively low extractive power which, however, is sufficiently active to remove the lower molecular weight mercaptans. In a second stage the partially de-mercaptanized hydrocarbon mixture is extracted with a second solution. The second solution preferably is an aqueous alkali metal hydroxide solution of relative high extractive power which is sufficiently active to remove the higher molecular weight mercaptans such as the C5 and higher mercaptans. 7

In practical operation the choice of most of those skilled in the art will be the relatively cheap and relatively easily handled'sodium hydroxide solutions common in the art while in the second step the more effective although more costly potassium hydroxide solutions re-enforced with an appropriate mercaptan solubility promoter such as isobutyric acid will be preferred. The two aqueous solutions are regenerated separately by passing an oxidizing gas through the aqueous solutions containing an organic oxidation promoter. The disulfldes obtained as a result of the oxidative conversion of the mercaptans (mercaptides) are separated and the regenerated solutions returned to their respective treating systems.

It'is to be noted that the novel process is more than a mechanical combination of operations old in the art in that the reagents used in the first stage could not be utilized in a single stage process to accomplish the entire object of the present invention in a single stage without very careful regeneration and the use of quite considerable quantities of reagent. On the other hand, if the reagents employed in the second step of the novel process were used in a single stage process for the complete accomplishment of the desired refining, not only would these same objections be valid, but it would be using a reagent of relatively greater capability than necessary'for a purpose wherein a cheaper reagent of less potentiality can be used with equal or better results.

Illustrative but not limiting is the following description of one embodiment of the novel process set forth in a. mor or less diagrammatic manner in Figure 2. I

Untreated oil, preferably devoid of substantial amounts of free or extractive hydrogen sulfide is introduced into extractor I at 3 through pipe 2. Active aqueous alkaline extracting solution is introduced into-extractor I at 4 by means of conduit 5. The oil rises in extractor I to be removed at 6 by means of pipe 1 while fat aqueous alkaline solution, i. e., containing mercaptans, is withdrawn from extractor I at 8 by means of pipe or conduit 9. The fat aqueous solution preferably is passed through heater II! where the temperature is raised to about 50 C. to C. It is to be noted that the oxidative conversion of mercaptans (mercaptides) to disulfides can be carried out at temperatures of about C. to 90 C. At lower temperatures the reaction is retarded and at temperatures above 90 C. there is a tendency for the reconversion oi disulfides to mercaptans to take place. Preferably extractor i is provided with a liquid level control (L. L. C.) II of customary and conventional structure whereby the upper level of the aqueous alkaline solution can be maintained fairly constant. Liquid level control ii is connected to valve 02 downstreamot heater is so that the valve I2 can be closed to raise the caustic level or opened to lower the caustic level in extractor I.

The heated fat solution in heater is is withdrawn through pipe Us to a mixer such as an orifice mixer I3 wherein the heated fat caustic solution is mixed with added oxidizing. gas, such as oxygen-containing oxidizing gas, for example air or oxygen, introduced from conduit it into pipe 828 at some point, such as it intermediate the heater It! and the orifice mixer it. The caustic solution and entrapped air are withdrawn from mixer or contactor it and introduced into the disulflde separator it by means of pipe i 7. Exhaust air is vented from the separator it by means of pipe id and disulildes withdrawn from the separator by means of pipe 09. The caustic solution which is regenerated by the conversion of the mercaptans to disulfldes is withdrawn from separator it by pump 2d through connection it. The regenerated alkaline solution is pumped to cooler 22 by pump it through connection 23. From cooler 22 the regenerated solution is returned to extractor i by pipe 5. The regenerated solution is cooled preferably to about 15 C. to about 35 C. in cooler iii.

The partially demercaptanized oil withdrawn from extractor i by means of pipe l is conveyed to extractor 2d of the second stage by pipe l and introduced therein at point 25. In extractor M the partially demercaptanized or desulfurized oil is extracted with a second aqueous alkaline solution introduced into extractor 2d at 25 by means of conduit 2?. The oil rises through the alkaline solution to be withdrawn as desulfurized oil at it through pipe 29. The fat caustic solution is withdrawn from extractor 2d at 3% by means of pipe M. The fat alkaline solution is passed through heater 32 and withdrawn therefrom by conduit 33. In the heater 32 the temperature of the fat alkaline solution is raised to not higher than about 90 C. The heated caustic solution is transferred to regenerator M by means of pipe as.

The extractor M is provided with a liquid level control (L. L. C.) 35 of conventional structure whereby valve 36 ma be opened to lower the gas may be used. The excess oxidizing gas is withdrawn from regenerator 34 by means of conduit 4 or separated from the reacted mate rials when separating the generated sulfides from the lean solution. Alternativel an open t'ank can be used. The regenerated caustic solution containing suspended disulfides is withdrawn from regenerator so and introduced into disulflde separator 42 by means of pipe 63. Disulildes are withdrawn from separator 42 by means of pipe M. Excess air is exhausted through vent 32 when, nevessary. Regenerated caustic reagent is withdrawn from separator 62 by means of pump W through pipe M. The regenerate In order that the extracting solutions em-' ployed in the first stage may be edectively regenerated by oxidative conversion of the mercaptans (mercaptides) to disulfides, it is necessary to have present in the solution at least during regeneration an organic oxidation promoter. Preferred among the organic oxidation promoters are those compounds having a functional or active polyhydroxy benzene structure. These compounds include polyhydroxy benzenes such as hydroquinone and pyrogallol, phenolic acids such as gallic acid and generally polyhydroxy benzene carboxylic acids and glucosides, esters, derivatives and condensation products thereof such as tannic acid and tannins. Particularly preferred among the organic oxidation promoters is tannin.

Tannins, as most chemists know, are complex materials the exact composition of which is not completely known. However, polyhydroxy benzene carboxylic acids such as the dihydroxy benzoic acids, including proto-catechuic acid and the resorcyclic acids, the trihydroxy benzoic acids such as gallic acid and its related compounds and derivatives are found useful. Nevertheless, a particular preference is had for those substances known as tannin or tannic acid. The term tannic acid or tannin is understood to cover those substances generally so grouped. Most oi them are substances widely distributed in the vegetable kingdom. They are soluble in water, possess an acid, astringent taste, are colored dark blue or green by ferrous salts, precipitate gelatin, and form leather when applied to animal hides. The constitution of many tannins is still somewhat obscure. Some appear to be glucosides of gallic acid, and decompose into gallic acid and sugar upon boiling with dilute acids. Others contain phloroglucin. Upon fusing with KOH these mostly form proto-catechuic acid and phloroglucin. Among the tannic acid compounds included in that term for the purposes of this invention there may be named gallotannic acid. digallic acid. gallyl-gallic acid and the various tannin materials derived from oak-bark, quinonebark, coffee, moringa-tannin, cutch, chestnut, logwood, sumac. and many other natural sources. Many such compositions appear to contain com pounds of the nature of polyhydroxy flavpinacoles, yielding, upon fusion with alkali, polyhydroxy benzenes such as phloroglucin, resorcinol. or pyrogallol, and polyhydroxy benzene carboxylic acids such as proto-catechuic or gallic such as oxygen or air and especially air.

ammo? acids. All such compounds are considered to be equivalent and embraced by the term tannic acid as herein used. A wide variety of vegetable tannins may be used satisfactorily. Illustrative of the type or tannins which may be employed are chestnut, wattle, quercitron, quebracho (cold water extract), tara. osage orange, logwood #4, ulmo, quebracho (hot water extract), spruce, oak, sumac, catch, and gambier. All of these compounds or mixtures may be employed and give regeneration at useful rates although the materials vary among themselves in emciency.

The aqueous alkaline solution employed as an extracting agent in the second stage is one having a higher extractive potentiality for mercaptans than the aqueous alkaline reagent employed in the first stage. Preferred among such solutions are those containing potassium hydroxide and a mercaptan solubility promoter such as isobutyric acid and solutions containing sodium hydroxide and sodium cresylate designated in the claims as sodium cresylate. these well known solutions the one containing potassium hydroxide and isobutyrate in a concentration of 6 normal and 3 normal respectively and another 5 normal 4 -to sodium hydroxide and 1.0 normal to sodium p-cresylate are preferred.

The aqueous alkaline reagent used in the second stage likewise contains organic oxidation promoter of which the preferred promoter is tannin. The oxidation promoter employed in the second stage caustic may be the same or a different promoter used in the first stage but it is preferred for practical reasons to use tannin in both solutions.

The tannin may be used in concentrations of about 0.1 per cent to about per cent but preferably in concentrations of about 0.3 per cent to about 1 per cent by weights To convert the mercaptans. (mercaptides) to disulfides for the regeneration of both solutions the fat solution containing the oxidation promoter is contacted with an oxidizing gas, particularly an oxygen-containing oxidizing gas, The mercaptan-sulfur'may be reduced by conversion to disulfide and separation thereof until the mercaptan-sulfur of the first stage solution is about 0.3 per cent by weight whilethe mercaptansulfur content of the second stage solution is about, 0.1 per cent to about 0.2 per cent by weight. However, for practical operation a minimum of 0.35 per cent by weight mercaptan-sulfur is considered to be the preferable minimum mercaptanscribed in conjunction with certain preferred embodiments thereof, those skilled in the art will understand that variations and modifications can be made. Such variations and modifications are to be considered within the purview of the specification and the scope of the appended claims. Furthermore, the phrase relatively low reactivity as used in the appended claims includes those mercaptans which are readily absorbed by. alkaline reagents known in the art to be weak or relatively poor absorption agents as compared to 6 normal potassium hydroxide-3 normal potassium isobutyrate (6 N KOH-3 N KEB) whereas the phrase "relatively high reactivity" as used in the appended claims .includes those mercaptans not readily absorbed by such alkaline reagents as 6 normal potassium hydroxide-3 normal potassium isobutyrate and 5 normal sodium hydroxide-1.0 normal sodium addition. the phrase "weak alkaline absorbent" as used in the appended claims includes those alkaline reagents which are relatively poor absorbents for mercaptans as compared to 6 normal potassium hydroxide-3 normal potassium isobutyrate. Similarly, the phrase "strong alkaline reagent" as used in the appended claims includes those alkaline reagents such as 6 normal potassium hydroxide-3 normal potassium isobutyrate and 5 normal sodium hydroxide-1.0 normal sodium p-cresylate.

I claim:

1. The method of removing mercaptans from a liquid mixture of hydrocarbons containing low boiling and high boiling mercaptans which comprises contacting in the absence of substantial amounts of gas containing free oxygen a liquid mixture of hydrocarbons containing low boiling and high boiling mercaptans with a first aqueous solution of alkali metal hydroxide substantially devoid of solutizer to extract low boiling mercaptans from said mixture of hydrocarbons and to produc 'a first fat" aqueous alkaline solution containing low boiling mercaptans and 0.

treated liquid mixture of hydrocarbons of substantially reduced content of low boiling mercaptans and containing high boiling mercaptans, contacting said treated liquid mixture of hydrocarbons in the absence of substantial amounts of gas containing free oxygen with a second aqueous solution of alkali metal hydroxide containing an amount of solutizer eflective to increase the solubility of high boiling mercaptans in said second aqueous solution of alkali metal hydroxide to tans, separating said disulfldes from said flrst aqueous alkaline solution to obtain a "lean" first aqueous, solution of alkali metal hydroxide containing said about 0.3 weight per cent mercaptansulfur. passing gas containing free oxygen through said second "fat" aqueous alkaline solution in the presence of a phenolic oxidation promoter, itself capable of inactivation by oxygen, to oxidize a portion of said high boiling mercaptans to disulfides and to leave unoxidized about 0.1 to about 0.2 weight per cent mercaptan-sulfur taining said about 0.3 weight per cent mercaptansulfur as mercaptides of low boiling mercaptans and with said second lean'f aqueoussolution of alkali metal hydroxide containing said solutizer \I and said about 0.1 to about 0.2 weight per cent mercaptan-sulfur.

2. The invention as set forth and described in claim 1 in which the second aqueous solution of alkali metal hydroxide is 6 normal to potassium 5 hydroxide and 3 normal to potassium isobutyrate.

3. The invention as set forth and described in claim 1 in which the second aqueous solution of alkali metal hydroxide is 5 normal to sodiumhydroxide and 1 normal to sodium cresylate. 10

STEPHEN P. CAULEY.

REFERENCES CITED Number '10 UNITED STATES PATENTS Name Date Rosenstein Oct. 20, 1931 Rosenstein Jan. 16, 1934 Pevere Sept. 17, 1935 Ridgway Oct. 20, 1936 Oberseider et a1. Nov. 10, 1936 Rogers et a1 July 4, 1939 Caselli et a1 June 9, 1942 Loyd Apr. 6, 1943 Loyd Apr. 6, 1943 Henderson et a1. Apr. 20, 1943 McNamara Nov. 30, 1943 Bond Feb. 20, 1945 Bolt Jan. 1'7, 1947 

